A. Field of the Invention
The present invention relates to SiC single crystals and production method thereof and more specifically, relates to SiC single crystals for preparing power device substrates by solution growth and to production method thereof.
B. Description of the Related Art
SiC has attracted attention as a material for power devices which have performance limits beyond that of Si power devices because SiC has a bandgap that is about three times higher than Si, a dielectric breakdown voltage that is about seven times higher than Si, and thermal conductivity that is about three times higher than Si. SiC is an ionic covalent crystal and crystallographically includes polytypes with single composition and various stacked structures relative to the c-axis direction. There are polytypes such as 4H (hexagonal crystal system having four molecules in one cycle), 6H (hexagonal crystal system having six molecules in one cycle), 3C (cubic system having three molecules in one cycle), and 15R (rhombohedron system having 15 molecules on in one cycle).
Different polytypes of SiC have different appearance ratio and different thermal physical properties such as thermal stability, bandgap, mobility and impurity level. Uniform single crystal substrates in which only one polytype exists are required in order to apply SiC for optical and electronic devices. In particular, 4H—SiC having a large bandgap is required for power devices.
Growth methods of SiC single crystals have been known such as sublimation, CVD and solution growth methods. The most frequently used method for production of SiC single crystals is sublimation because the liquid phase does not exist at ordinary pressure. In sublimation, highly pure SiC powder is heated at 2200° C. to 2500° C. and the sublimed raw material is fed to the surface of a seed crystal which has been brought to a temperature lower than the powder, thereby allowing re-crystallization. Because various chemical species composed of Si and C are mixed in the sublimed gas and complex reactions occur, polymorphic transition easily occurs and lattice defects such as dislocation are produced. As dislocation causes leaks when PN diodes are produced, it is desired to reduce dislocation density (EPD).
In the CVD method, a single crystal seed crystal substrate is simultaneously fed with diluted carbohydrate gas and silane gas and SiC single crystals are grown epitaxially at the substrate surface by chemical reaction. In CVD method, balanced etching and deposition promote growth. Therefore, the growth rate is slow and it is not suitable for the production of bulk single crystals, and this method is mainly used as an epitaxial growth method of drift layers.
In solution growth, a melt containing Si and C is brought into contact with seed crystals having a lower temperature than that of the melt so as to allow supersaturation of SiC in the melt, thereby allowing growth of SiC single crystals on the surface of seed crystals. In the solution growth method, single crystals having high quality can be obtained because they have less lattice defects and less crystal polymorphism compared to other growth methods. However, at a practical temperature of 2000° C. or lower, solubility of C in a Si melt is extremely low, causing a lower growth rate.
Japanese Patent Application Laid-open No. 2004-002173 discloses a solution growth method in which a single crystal is grown on a seed crystal substrate from a melt of an alloy comprising Si, C and M (wherein M is either Mn or Ti), wherein an atomic ratio between Si and M is Si1-xMx wherein 0.1≦x≦0.7 when M is Mn and 0.1≦x≦0.25 when M is Ti.
Japanese Patent Application Laid-open No. 2007-261843 discloses a method of SiC single crystal growth on a SiC seed crystal substrate by contacting the seed crystal substrate with a melt comprising Si, C, V and Ti wherein an atomic ratio between Si and V represented by [V]/([Si]+[V]) satisfies the relation 0.1≦[V]/([Si]+[V])≦0.45 and an atomic ratio between Si and Ti represented by [Ti]/[Si]+[Ti]) satisfies the relation 0.1≦[Ti]/[Si]+[Ti])≦0.25, and supercooling the melt in the vicinity of the seed crystal substrate to allow supersaturation of SiC in the melt.
Japanese Patent Application Laid-open No. 2007-076986 discloses a method of SiC single crystal growth on a seed crystal substrate for SiC growth by contacting the seed crystal substrate with a melt comprising Si, Ti, M (M: any one of Co, Mn and Al) and C, wherein an atomic ratio between Si, Ti and M is SixTiyMz wherein 0.17≦y/x≦0.33 and 0.90≦(y+z)/x≦1.80 when M is Co or Mn and 0.17≦y/x≦0.33 and 0.33≦(y+z)/x≦0.60 when M is Al, and supercooling the melt in the vicinity of the seed crystal substrate to allow supersaturation of SiC in the melt.
According to the method disclosed in Japanese Patent Application Laid-open No. 2004-002173 in which Mn or Ti is added, the C concentration in the melt at 2000° C. or lower can be increased compared to Si—C dual systems so that the crystal growth rate can be increased. Especially, addition of Ti allows the production of SiC single crystals having high quality with a few times higher growth rate than the conventional liquid phase growth from Si—C dual system melts. However, although addition of Ti allows stable growth with respect to the growth of 6H—SiC single crystals, 6H—SiC polytype is mixed during the growth of 4H—SiC single crystals which are promising for power device applications, so that stable growth cannot be obtained. In addition, the C concentration in the melt is significantly increased by addition of Mn; however, stable growth cannot be obtained for a prolonged time because the vapor pressure of Mn is low.
According to the methods disclosed in Japanese Patent Application Laid-open No. 2007-261843 and Japanese Patent Application Laid-open No. 2007-076986, the Si—Ti—C triple system is further added with V, Co, Mn or Al to increase the C concentration of the melt and liquid phase growth is carried out by using the melt having a higher C concentration. However, neither method provides the crystal growth rate comparable to that of sublimation regarding the growth of 4H—SiC single crystals.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.